1. Field of the Invention
The present invention relates to a heat shut off condition determination method and apparatus for optical recording and an optical recording method and apparatus.
2. Related Background Art
At present, optical recording is achieved by exclusively utilizing a thermal nature of a laser beam, and recording media (optical disks) include (1) a write-once type optical disk (pit formation type) allowing recording only once, such as an optical disk having a thin metallic film or thermet film as a recording layer, and (2) an optical disk which allows repeated recording, reproduction, and erasure, such as a magnetooptical disk having a magnetic thin film as a recording layer, a phase-change optical disk having a metallic film or thermet film as a recording layer, which film causes a phase change between crystal and amorphous phases, and the like.
Several ten thousands of tracks on which information is to be recorded are spirally or concentrically formed on an optical disk. Two types of information units corresponding to "0" and "1" are formed on each track, thereby recording information. In practice, the track itself (i.e., a background portion) indicates a first information unit corresponding to one of "0" and "1", and second information units (called marks, recently) corresponding to the other one of "0" and "1" are formed on the track in a point or island pattern. In this case, the presence/absence of marks, the mark interval, the mark length, the mark formation start position (i.e., the leading edge position of a mark), the mark formation end position (i.e., the trailing edge position of a mark), and the like express information. In particular, a method of expressing information by the edge position of a mark is called mark length recording.
An optical recording apparatus is mainly constituted by a laser source, a radiation optical system for radiating a laser beam emitted from the laser source onto an optical disk, modulation means for modulating the laser beam intensity according to information to be recorded, and optical disk rotation means. In a magnetooptical recording apparatus, magnetic means for applying a bias magnetic field to the radiation position of the beam is added.
Since optical recording exclusively utilizes a thermal nature of a laser beam (heat mode), the laser beam intensity need only be pulse-modulated between a relatively high first level and a relatively low base level (second level) in principle. When the laser beam intensity is at the first level, a mark is formed; when it is at the second level, no mark is formed. That is, one mark is formed in correspondence with one pulse. The second level can be zero since it does not form any mark. However, when a mark is to be formed, in other words, when the leading edge of a mark is to be formed, it is preferable that the disk temperature state immediately before formation be always positively maintained in a constant temperature state. Otherwise, the leading edge position varies depending on the temperature state immediately before formation. Such a variation disturbs high-density recording. Thus, it is preferable that an optical disk be pre-heated to a predetermined temperature .THETA..sub.pre, i.e., be set in a pre-heat state, and the second level be normally set at an intensity P.sub.pre for maintaining this pre-heat state (temperature .THETA..sub.pre). The temperature .THETA..sub.pre allows the disk temperature immediately before mark formation to be constant independently of the peak temperature position of the beam or the data pattern recorded at the spot center position, and P.sub.pre is given by the following formula: EQU .THETA..sub.pre =A.times.P.sub.pre .times.{1-exp(-.infin./.tau.)}+.THETA.A(formula 1)
where A (.degree.C./mW) is the heat efficiency of the laser beam intensity determined by the disk, the spot, and the recording line density, and .THETA.A (.degree.C.) is the disk temperature in a non-radiation state of the beam.
The first mark formation is a method of forming one mark in correspondence with one pulse. FIG. 11 is a waveform chart of the laser beam intensity when one mark is formed by the first method. As shown in FIG. 11, a pulse waveform for raising the laser beam intensity from the base level (second level) P.sub.pre to start mark formation, and after the raised intensity (first level) P.sub.W1 is maintained for a time T.sub.W1 by a half-width, reducing the intensity to P.sub.pre is used. In this case, when the mark length is large, an adverse effect due to heat accumulation appears. The adverse effect is that even when the laser beam intensity is reduced to P.sub.pre to end mark formation, the medium temperature cannot be easily decreased to the mark formation start temperature or less due to the heat accumulation so far. For this reason, the mark length or width becomes unexpectedly large. This adverse effect is called "recording data pattern dependency of the mark formation end position, i.e., the mark trailing edge position". This dependency disturbs high-density recording, and decreases identifiability of data.
The second mark formation method can solve this problem to some extent. FIG. 12 is a waveform chart of the laser beam intensity when one mark is formed by the second method. As shown in FIG. 12, the intensity of the laser beam to be radiated onto the optical recording medium is raised from P.sub.pre to an intensity P.sub.W1 higher than P.sub.pre, and after P.sub.W1 is maintained for a time T.sub.W1, the intensity is reduced to an intensity P.sub.LT lower than P.sub.W1. Thereafter, the intensity is modulated between P.sub.LT and an intensity P.sub.W2 higher than P.sub.LT. The time for maintaining P.sub.W2 is T.sub.W2, and the modulation period upon intensity modulation between P.sub.LT and P.sub.W2 is T.sub.p. This method is called a pulse train method since a waveform (see FIG. 11) which originally consists of one pulse consists of a small start pulse, and one or two or more following small pulses. In this case, the temperature at the laser beam radiation position on the optical disk during mark formation normally drifts up and down near a high temperature.
On the other hand, in the case of high-density recording, the start position of the next mark undesirably varies depending on the end position of the immediately preceding mark. This phenomenon is called "recording data pattern dependency of the mark formation start position, i.e., the mark leading edge position". In order to solve this problem, an optical recording method for, when the laser beam intensity is reduced to end mark formation, temporarily reducing the intensity to P.sub.LB lower than P.sub.pre, and after an elapse of a time T.sub.off, raising the intensity to P.sub.pre has been proposed. FIGS. 9 and 10 are waveform charts of the laser beam intensity when one mark is formed by this method. FIG. 9 shows a case wherein one mark is formed in correspondence with one pulse, and FIG. 10 shows a case wherein a mark is formed by the pulse train method. In this method, formation of the next mark is started from a predetermined position regardless of the length of the immediately preceding mark. In this manner, a thermal influence from the immediately preceding mark is shut off for the next mark. A condition for shutting off the thermal influence in this manner is called a "heat shut off condition", and is expressed by P.sub.pre, P.sub.LB, and T.sub.off. Conventionally, in the heat shut off condition, as described in STANDARD ECMA/TC31/92/36 3rd Draft Proposal, p. 87 (see FIG. 13), European Computer Manufacturers Association (to be abbreviated to ECMA hereinafter), P.sub.LB is fixed at a laser beam intensity P.sub.r in a reproduction mode, and T.sub.off is fixed to a write clock period T.